Heretofore, many prior attempts have been made to utilize solar energy for commercial and residential heating purposes. However, many of the prior developments have not been practical or efficient in the utilization of solar energy, and considerable attention needs to be given to the problem of utilizing solar energy efficiently.
It is also recognizable that conversion from conventional heating systems to heating systems utilizing solar energy as the sole source of heat are not practical at present. Therefore, it is necessary that some efficient and practical combination of conventional and solar heating systems be utilized until technology advances to the stage where solar energy as the sole source of heat is practical.
Energy conservation requirements necessitate that solar energy be utilized as much as possible within the realm of practicality and efficiency. One form of conventional system heating system to which the application of solar energy appears attractive is the conventional heat pump system. Prior heat pump systems generally employ an outdoor heat exchanger in heat exchange relation with ambient air and an indoor heat exchanger in heat exchange with the interior space to be heated. Heat is absorbed from the outdoor air and pumped to the interior space when heating is required. The heat pump system may be operated in the reverse mode to cool the interior space during the summer time.
However, during severe winter conditions, prior heat pump systems may be unable to provide satisfactory heating due to the reduced capacity of the outdoor heat exchange coil at very low ambient temperatures. The capacity and efficiency of the heat pump system decreases as the temperature difference between the indoor and outdoor heat exchanger increases. The volumetric efficiency of the compressor becomes less as the outdoor temperature drops. In other words, when the outdoor temperature drops to a certain point, the heat pump system can no longer pump heat content from the air in an efficient manner compared to the work done by the compressor. The density of the refrigerant gas is less at lower outdoor temperatures which results in a lower volume of refrigerant being pumped and consequently a lower quantity of heat being transferred from the outdoor air.
Prior devices have been developed which utilize a supplementary source of heat to maintain the main heat source temperature at a high level to overcome the foregoing problems. Such an arrangement is shown in U.S. Pat. No. 3,563,304 wherein a first portion of the outdoor heat exchange coil is disposed in heat exchange relationship with the ambient air and a second portion of the outdoor coil is in heat exchange relationship with a pool of water. During the heating mode, the refrigerant is delivered through the first portion of the outdoor coil to absorb heat from the ambient air and then in series through the second portion of the outdoor coil where the unevaporated remainder of the refrigerant is vaporized by absorption of heat from the water in the tank. The refrigerant vapors then pass to the compressor for compressing the refrigerant. During periods of extremely cold ambient conditions, the second portion of the coil immersed in the water pool provides substantially all the heat for exchange with the interior of the house. The water in the pool from which the heat is absorbed is utilized down to the freezing point of the water, and thereafterwards the latent heat of fusion of the water is utilized as the heat source to provide heating to the building. An electrical heater is immersed in the water pool to melt any ice in the tank during periods of low power demand when rates are lower. The electrical heater raises the temperature of the water just above the freezing point. During extremely cold conditions, the first portion of the outdoor heat exchange coil is taken out of operation by de-energizing the fan which blows ambient air over the coil. However, the first portion remains in series with the second portion so that some heat is lost in the first portion.
U.S. Pat. Nos. 3,194,303 and 2,689,090 show heat pump systems which realize a supplemental heat source such as solar energy wherein a heat transfer fluid is circulated in series through a solar heat exchange coil and an underground heat exchange coil. The heat transfer fluid is then passed in heat exchange relationship with the refrigerant of the system and the heat of the fluid is transferred to the refrigerant prior to being compressed for vaporizing the refrigerant. However, the refrigerant is not pumped directly in heat exchange relationship with the heat sources and the heat exchange coils associated with the heat sources are in series.
U.S. Pat. No. 2,428,876 discloses a heat pump system having its evaporator coil immersed in a storage container having a fluid contained therein which is heated by direct solar energy. However, there is no supplemental heating coil to compensate for extended periods of cloudiness.